1. Field of the Invention
The present invention relates to a waveguide type optical element, an integrated optical waveguide type element using the waveguide type optical element, and a method of manufacturing the waveguide type optical element and the integrated optical waveguide type element using the waveguide type optical element. More particularly, the present invention relates to a method to improve high speed modulation characteristics by controlling diffusion of p type impurity around a light absorption layer, to the light absorption layer.
2. Description of the Related Art
As a related art, there is a reference “Analysis of low voltage/high output in a DFB-LD/modulator integrated source, Singaku Giho QEL95-18 (1995-06)”.
Conventionally, a method of performing bury growth has been proposed as a method of forming a window structure to lower a reflectivity on an end face ratio as disclosed in the above reference with respect to waveguide type optical element, in particular, a semiconductor laser with an electro absorption modulator.
Below, a detailed explanation will be described referring to FIGS. 10A-10D.
As shown in FIG. 10A, a grating 2 is partially formed on an InP substrate 1. A pair of selection growth masks 3 is formed on the InP substrate 1. Mask width of the pair of selection growth masks 3 in an area at the side of formation of the grating 2 is wider than mask width of the pair of selection growth masks 3 in the other areas. A multi quantum well layers 4 including a waveguide layer and an activation layer, a light absorption layer 5, and a first clad layer 6 are sequentially select-grown on the InP substrate 1 where the pair of selection growth masks 3 by using a metal organic vaper phase (MOVPE) epitaxy method. An interval of the pair of selection growth masks 3 is about 1 μm to 30 μm and mask width is about 5 μm to 50 μm. p As shown in FIG. 10B, the multi quantum well layers 4, the light absorption layer 5, and the first clad layer 6 are strip-like etched by using an insulation film mask 7.
As shown in FIG. 10C, a second clad layer 8 and a contact layer 9 are formed.
As shown in FIG. 10D, a p type contact electrode 10 for an activation area, and a p type contact electrode 11 for a modulator are deposited. After an n type electrode 12 is deposited, each of electrodes 10, 11, and 12 is alloyed by anneal treatment. After a chip is cloven, a low reflection film 13 is coated.
Each length of a DFB laser area 14, a modulator area 15, and a window area 16 are about 300 μm to 700 μm, 50 μm to 250 μm, and 10 μm to 50 μm, respectively, in a light propagation direction in the conventional waveguide type optical element formed by the above steps.
An absorption coefficient of a light absorption layer is increased and light absorption is increased by applying a reverse voltage to the modulator area in the conventional waveguide type optical element. Increase amount of the absorption coefficient depends on an applied electric field and change amount by the electric field is determined by a structure of the absorption layer. The electric field is proportional to the voltage and is inversely proportional to thickness of a depletion layer. Since frequent characteristics at a time when a high speed modulation signal voltage is applied to a modulator depends on an electrical capacity of an absorption layer, the electrical capacity has to be reduced in a case where high speed modulation characteristics (more than 10 GHz) is required. Therefore, an absorption layer area has to be reduced or thickness of a depletion layer has to be thick. As described above, relation with a structure parameter to increase the absorption layer is a trade off. Not only an electro absorption layer but also thickness of the depletion layer has to be controlled in high speed modulation characteristics of the modulator.
However, crystal growth is performed by the metal organic vaper phase (MOVPE) epitaxy method in a method of forming a conventional waveguide type optical element. In this method, Zn (Zn) is used as p type impurity. This Zn has a feature in which the diffusion coefficient is large and is easily diffused. Since modulation characteristics are changed by Zn diffusion, an accurate control is needed. Especially, when a high frequency signal is supplied, an electric signal is leaked to the waveguide side and the electric capacity is reduced in a case where Zn applied at a time where ridge waveguide growth is diffused to the absorption layer in an inverse mesa ridge waveguide structure which the present invention concerns. Thereby, modulation characteristics are also deteriorated.